Adsorption of Ni(SO4) on Malaysian rubber-wood ash

Shameem Hasan, Mohd. Ali Hashim, Bhaskar Sen Gupta *

Institute of Post Graduate Studies and Research, University of Malaya, 50603 Kuala Lumpur, Malaysia

Received 1 December 1998; received in revised form 11 June 1999; accepted 20 June 1999

Abstract

The possible use of wood ash as an adsorbent of nickel sulphate from dilute solutions and the e�ect of operating parameters were

investigated in this study. The rate constants of adsorption were determined at di�erent concentrations and temperatures. The

applicability of the ®rst-order reversible equation and an empirical kinetic model were tested to understand the kinetics of nickel

sulphate removal at di�erent concentrations. Pore di�usion was found as the rate-controlling step. The Langmuir and Freundlich

isotherms were applied to ®nd out the adsorption parameters. The activation energy of adsorption was ÿ11.54 kJ molÿ1. The value

of the enthalpy change was DH�ÿ10.35 kcal molÿ1. Ó 1999 Elsevier Science Ltd. All rights reserved.

Keywords: Wood-ash; Adsorption; Kinetics; Activation energy

1. Introduction

Pollution in the environment by heavy metals hasreceived attention due to the greater understanding oftheir toxicological importance in the ecosystem, agri-culture and human health. Industrial wastewatersgenerated in the metal ®nishing and electroplating in-dustries typically contain toxic heavy metals such ascopper, cadmium, nickel, lead, zinc, chromium, etc.The plating operation consists of several stages, whichproduce a signi®cant amount of wastewater. The met-als and chemicals present in the electroplating waste-water in soluble form are in general toxic (Rakmi,1996). Many methods have been proposed for the re-moval of heavy metals. Chemical precipitation, alum oriron coagulation, membrane ®ltration, ion exchangeand adsorption are some of the most commonlypracticed processes (Patterson, 1985). Safe and e�ectivedisposal of wastewater containing heavy metals is al-ways a challenging task for the industries due to thefact that cost-e�ective treatments are not readilyavailable.

Adsorption is an e�ective and important means ofcontrolling the extent of pollution due to metallic speciesin industrial e�uent. A number of reports (Huang,

1978; Jung and John, 1977; Young and Peter, 1987;Peters et al., 1985) are available on the removal of metalions from wastewater using activated carbon as an ad-sorbent. Although activated carbon is suggested (Huangand Blankenship, 1984) as a competitive and e�ectiveprocess for the removal of heavy metals, in developingcountries naturally occurring and abundantly availablebiosorbents are receiving signi®cant attention due totheir low cost (Pandey et al., 1985). Materials such asChinese peat (Zhipei et al., 1984), activated sludge(Venkobachar, 1990) waste slurry generated from afertilizer plant (Srivastava et al., 1989), ¯y ash fromthermal power plant (Pandey et al., 1985), palm pressedleaves (Tan et al., 1993), agrowaste (Sharma and For-ster, 1994a,b,c), Giridih coal and crushed coconut shell(Bhattacharya and Venkobachar, 1984) have been triedas low cost adsorbents.

The present work describes the batch adsorptioncharacteristics of nickel sulphate on rubber-wood ash.The e�ects of various operating parameters were alsoinvestigated. The concentration of nickel sulphate re-sembled the range of concentrations found in electro-plating wastewater in Malaysia (Department ofEnvironment, 1985). Rubber plants are cultivated inMalaysia on a commercial scale. There is an abun-dance of rubber plant waste from the logging andfurniture industries. The waste is used as fuel by small-scale industries as an alternative source of energy. Theash is disposed for land®ll purposes and is available inbulk.

Bioresource Technology 72 (2000) 153±158

* Corresponding author. Tel.: +60-3-7594409; fax: +60-3-7568940.

E-mail address: bhaskar_sengupta@yahoo.com (B.S. Gupta)

0960-8524/00/$ - see front matter Ó 1999 Elsevier Science Ltd. All rights reserved.

PII: S 0 9 6 0 - 8 5 2 4 ( 9 9 ) 0 0 1 0 1 - 7

2. Methods

The rubber-wood (Hevea brasiliensis) ash sampleswere prepared by grinding wood charcoal in a labora-tory hammer mill (IKA, Germany). The samples werethen incinerated in a furnace (Carbolite, UK) at 600°Cfor 4 h. The average value of the combustible carbon inthe sample was about 0.38%. The particles were sievedthrough a 53 l sieve. The sieved particles (10 g) weresoaked in 400 ml of 0.5 M HNO3 for 4 h and then ®l-tered and rinsed with distilled water until the ®ltrate wasbetween pH 6.8 and 6.9. After washing, the particleswere dried overnight at 60°C. The average size of theparticles was 6 lm measured by a particle size analyzer(Counter, USA; Model No. LS230). Metal stock solu-tion was prepared by dissolving analar gradeNiSO4 á 6H2O (Fluka Chemie; Germany) in ultrapurewater.

The experiments were carried out using batch tech-niques. Wood-ash (1.0 g) was suspended in 50 ml ofmetal solution with initial metal ion concentrationsranging from 0.345 mmol (20 mg/l) to 2.07 mmol (120mg/l). The pH of the metal solution was adjusted to 5 bythe addition of 0.1 M NaOH or 0.1 M HNO3. Two typesof control ¯ask, i.e., ash-free control and metal-freecontrol were prepared. The ¯asks were then incubated ina shaker (New Brunswick Scienti®c, USA) at 30°C andagitated at 120 rpm for 4 h. The mixtures were thencentrifuged (Sigma 2±15, Germany) at 4500 rpm for 15min to separate the supernatant from the biomass. Thecontrols were treated in the same way. The supernatantwas then ®ltered and analyzed for nickel concentrationsby the inductively coupled plasma spectrometry method(Baird ICP 2000 Spectrometer, USA). All runs werereplicated and the average of two values was used in thecalculations. The maximum di�erence between the twovalues was less than 5% of the mean.

3. Results and discussion

3.1. E�ect of nickel sulphate on wood ash contact time andconcentration

Adsorption is a slow process and adequate contacttime is necessary to allow the system to approachequilibrium. In this work, the adsorption capacity ofwood ash and contact time were studied. The removal ofnickel from aqueous solution by adsorption increasedwith time (Fig. 1): signi®cant adsorption occurred in60 min and equilibrium was attained in approximately120 min in most cases.

The concentration vs. time plots shown in Fig. 1 in-creased monotonically to saturation at various concen-trations of Ni (II). This indicates the possibility of a

monolayer adsorption of nickel on the wood ash (Singhet al., 1984).

4. Adsorption kinetics

The adsorption of Ni (II) from liquid phase to solidphase can be considered to be a reversible process(Bhattacharya and Venkobachar, 1984). The kinetics ofNi (II) adsorption on wood ash was studied in the lightof a ®rst-order reversible equation (Yadava et al., 1988)is given by Eq. (1).

log�qe ÿ q� � log qe ÿ �k0=2:3�t; �1�

where, qe is the amount of Ni (II) adsorbed (m mol/g) atequilibrium, q the amount of Ni (II) adsorbed (m mol/g)at any time t, k0 � �k1 � k2� minÿ1, and k1 and k2 arede®ned as forward and reverse rate constants of the ®rst-order reversible reaction, t is the adsorption time inminutes.

The values of k1; k2 and k0 were calculated from theslopes of the plots log (qeÿ q) vs. t at 20°C, 30°C and40°C, respectively at di�erent nickel concentrations andare given in Table 1. The rate constants of intraparticletransport (kid) for di�erent concentrations of solution atdi�erent temperatures were determined from the slopesof the linear plots t1=2 vs. amount of Ni (II) adsorptionat 20°C, 30°C and 40°C (Weber and Morris, 1963;Yadava et al., 1988). These values are listed in Table 1.The value of the pore-di�usion rate constant (kid) in-creases with the increase in initial adsorbate concentra-tion and decreases with rise in solution temperature.This is consistent with the observation of Yadava et al.(1988) for adsorption of Pb (II) on ¯y ash.

The activation energy was determined (Tewary et al.,1972) from the slope of the Arrhenius plot as shown inFig. 2 and was found to be ÿ11.54 kJ mol.ÿ1

Fig. 1. E�ect of time and concentration on Ni(II) removal by rubber-

wood ash. The concentrations of Ni(II) were (o) 2.07, (´) 1.21 and (h)

0.345 mmol/l, respectively. (Conditions: Temperature).

154 S. Hasan et al. / Bioresource Technology 72 (2000) 153±158

5. Empirical kinetic model

The adsorption data obtained from the batch studieswere analysed using the following empirical modelavailable in the literature (Prakash et al., 1987; Yadavaet al., 1988).

log�t � 1� � B�Ci ÿ C�A; �2�where, Ci is the initial concentration of Ni (II) in watermmol/l, C the concentration of Ni (II) in water at time t,in mmol/L, t the time in minutes and B and A the em-pirical constants dependent on Ci.

The values of B and A were determined from theslopes and intercepts of the linear plots log[log(t + 1)] vslog (CiÿC). Eq. (2) can be correlated to Ci as follows:

For Ci � 2:07 mmol=l;

log �t � 1� � 1:424� �Ci ÿ C�1:2452�3�

and for Ci � 1:21 mmol=l;

log �t � 1� � 1:9364� �Ci ÿ C�0:2547:

�4�

Eqs. (3) and (4) show that the empirical constant Bvaries considerably with the initial concentration ofnickel sulphate, whereas A decreases with the decrease ininitial concentration of Ni (II). The empirical constantsA and B are functions of the initial concentration of Ni(II) in solution (Ci) (Prakash et al., 1987; Yadava et al.,1988).

6. Adsorption isotherm

The adsorption data can be interpreted using severalcorrelations, which describe the distribution of the sol-ute between the liquid phase and the solid phase. Theequilibrium data for the adsorption of Ni (II) on wood-ash at 30°C were ®tted to the Langmuir isotherm asindicated in Fig. 3. The equilibrium data were also ®ttedto the Freundlich isotherm as shown in Fig. 4. The valueof the empirical constants K and n were found to be0.699 and 3.70, respectively at 30°C. The values of ad-sorption constants at various temperatures (20±40°C)for Langmuir isotherm are listed in Table 2.

Table 1

The values for rate constants for ®rst-order reaction and pore di�usion

Rate parameters at di�erent concentrations (pH 5; Temp. 30°C) Rate parameters at di�erent temperatures (2.07 mmol/l)

Concent-

ration

(mmol/l)

k0

(minÿ1)

k1 ´ 102

(minÿ1)

k2 ´ 103

(minÿ1)

kid

(mmol/g

min1=2)

T (°C) k0 (minÿ1) k1 ´ 102

(minÿ1)

k2 ´ 102

(minÿ1)

kid

(mmol/g

min1=2)

2.07 0.035 2.27 12.25 0.59 20 0.0409 2.73 13.6 0.61

1.21 0.0306 2.53 5.3 0.42 30 0.035 2.27 12.26 0.59

0.345 0.043 4.19 1.25 0.08 40 0.0316 1.92 12.4 0.55

Fig. 2. Arrhenius plot for the determination of activation energy.

Fig. 3. Langmuir plot at 30°C temperature. Q0, b are Langmuir

constants.

Fig. 4. Freundlich isotherm plot at 30°C temperature. K, n are em-

pirical constants.

S. Hasan et al. / Bioresource Technology 72 (2000) 153±158 155

7. Adsorption at various temperatures

Solution temperature exerts a signi®cant in¯uence onthe rate of adsorption (Yadava et al., 1988). The resultsindicate that the adsorption capacity of wood ash withrespect to Ni (II) decreases with increase in temperature,indicating that the adsorption process is exothermic innature (Yadava et al., 1988). The Langmuir isothermcan be characterized by a dimensionless separationfactor R, which is de®ned as follows (Namasivayam andRanganathan, 1993):

R � 1=�1� Q0Ci�; �5�where Qo is the Langmuir constant, and Ci the initialsolute concentration (mmol/g).

The value of R indicates the shape of the isotherm asexplained in Table 3 and it also shows the values of Rfor the adsorption isotherms of nickel on wood ash. Foradsorption of nickel on wood ash, the values of R arefound to be less than 1 and greater than 0. This indicatesa favourable adsorption of nickel on wood ash in thetemperature range of 20±40°C.

The net change in enthalpy of adsorption DH isrelated to the Langmuir constant b as follows (Pandeyet al., 1985):

lnb � lnb0 ÿ DH=RT ; �6�where, b0 is a constant.

The value of DH for the adsorption of Ni (II) onwood ash was calculated from the slope of the plot ln bvs. 1/T (Fig. 5) and was found to be ÿ10.35 kcal molÿ1.

8. Thermodynamic parameter

The various thermodynamic parameters have beencalculated using the following equations:

DG0 � ÿRT ln�Kc�; �7�

DH 0 � T2T1

T2 ÿ T1

lnKc2

Kc1

; �8�

DS0 � DH 0 ÿ DG0

T; �9�

where DG0 is the change of free energy, DH 0 the changein enthalpy, DS the change in entropy.

The equilibrium constants, Kc;Kc1and Kc2

(ratios ofK1 and K2 at temperatures 20°C, 30°C and 40°C) wereobtained from Table 1.

Table 4 indicates free energy change (DG0), theenthalpy change (DH 0) and entropy change (DS0) foradsorption of Ni (II) on rubber-wood ash at di�erenttemperatures. The negative values of free DG0 indicatethe spontaneity of the adsorption process and those ofDH 0 show the exothermic nature of the adsorptionprocess. Yadava et al. (1988) and Pandey et al. (1987)suggested that the negative entropy change, DS0 wouldindicate faster interaction with the active surface sites ofadsorbent (forward reaction).

Table 2

The value of the Langmuir constants at di�erent temperatures

Temperature Q0 b Ce/qe

(°C) (mmol/g) (mmol)

20 0.0424 1.678 14.01 + Ce/0.042

30 0.0492 1.426 20.33 + Ce/0.0492

40 0.053 1.244 19.00 + Ce/0.053

Table 3

Shape of isotherm on the basis of R and equilibrium parameter R for

Ni(II) on rubber-wood ash

Value of R Type of isotherm Temperature

(a) Shape of isotherm on the basis of R

R > 1 Unfavourable

R� 1 Linear

0 < R < 1 Favourable

R� 0 Irreversible

(b) Equilibrium parameter R for Ni(II) on rubber-wood ash

0.991 20°C

0.908 30°C

0.904 40°C

Fig. 5. Plot for the calculation of apparent heat of adsorption.

Table 4

Thermodynamics parameters at di�erent temperatures

Temperature

(°C)

Thermodynamic parameters

ÿDG0 ÿDH0 ÿDS

20 1.341 3.823 8.47

30 1.256 3.799 8.393

40 1.1732 ± ±

156 S. Hasan et al. / Bioresource Technology 72 (2000) 153±158

9. Adsorption at various pH values

The e�ect of the pH on the adsorption of nickel onwood ash was studied by varying the pH of the sorptionmixture in the range 2±8. The pH was adjusted by ad-dition of 0.1 M HCl or NaOH. The pH of the solution isan important factor in determining the rate of surfacereactions, with the initial pH of the solution havingmore in¯uence than the ®nal pH. The e�ect of pH on theadsorption of nickel on wood ash may be explained onthe basis of an aqua-complex formation owing to theoxides present in the ash (Rao et al., 1992). A positivecharge develops on the surface of the oxides of the ash inan acidic medium as follows:

MO�HOH!MOH�2 �OHÿ

The e�ect of pH on the adsorption of nickel on woodash is shown in Fig. 6. It was found that the removal ofnickel by adsorption had increased with the increase ofpH from 2 to 5 at 30°C temperature. Similar observationwas made by Low et al., 1995. The critical pH was foundto be between 4.5 and 5.5. As pH decreased from 5 to4.5 and further, the adsorption peak declined with thedecrease in pH. The variation of adsorption of nickel atvarious pH can be explained on the basis of metalchemistry in solution and the surface chemistry of thewood ash. The pHmax where maximum adsorption ofthe metal takes place seems to be related to the pK or the®rst hydrolysis product of the metal. The exact natureand distribution of hydroxo complexes depend onthe concentration of the ligands i.e. solution pH and onthe soluble metal concentration. This implies that thevariation in adsorption capacity in this pH range islargely due to the in¯uence of pH on the surface ad-sorption characteristics of the wood ash.

It is believed that the surface charge of the ash be-comes negative as a result of deprotonation of surfacefunctional groups with the increase in pH. Conse-quently, the coulombic attraction between positively

charged nickel ions and negatively charged wood ashwould increase.

Various theories have been put forward to describeand interpret metal ion interaction at solid±solutioninterface. According to Mc Naughton and James (1974),nickel is removed from aqueous solution by the fol-lowing mechanisms:

(a) ion exchange reaction,(b) metal ion adsorption at hydrated oxides of thesurface,(c) metal hydroxyl species adsorption at the hydratedoxide surface.

The ion-exchange reaction in fact di�ers from adsorp-tion. Since nearly every ion exchange process is ac-companied by adsorption and desorption, adsorptionsometimes is indistinguishable from ion exchange.

In view of the above fact, it seems that the mechanism(a) and (b) are more e�ective than (c) for the nickel-ashsystem because of the decrease in adsorption above pH5.5. The decrease in adsorption of Ni (II) above pH 5.5may be due to the formation of Ni(OH)2. This is largelydue to the fact that substantial precipitation of nickel asnickel hydroxide occurs at high pH values. The forma-tion of hydroxide precipitate reduces the amount of freenickel ions, which can bind to the ash.

10. Conclusion

Rubber-wood ash appears to be a suitable adsorbentfor the removal of nickel from dilute solution. An ex-tensive experimental investigation was conducted to es-tablish the adsorption behavior of wood ash. The extentof metal removal depends signi®cantly on the pH values.

The adsorption reaction can be described by ®rst-order reversible reaction. The equilibrium data can beapproximately described by both Langmuir andFreundlich isotherms. The equilibrium data obtained atpH 5 and 30°C indicate that the maximum adsorptioncapacity of the rubber-wood ash for nickel is 0.492mmol/g.

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